A. V. Oza, J. V. Seidel, H. J. Hoeijmakers, A. Unni, A. Y. Kesseli, C. A. Schmidt, T. Sivarani, A. Bello-Arufe, A. Gebek, M. Meyer zu Westram, S. G. Sousa, R. M. C. Lopes, R. Hu, K. de Kleer, C. Fisher, S. Charnoz, A. D. Baker, S. Halverson, N. Schneider, A. Psaridi, A. Wyttenbach, S. Torres, I. Bhatnagar, R. E. Johnson
Abstract
Neutral sodium (Na I) is an alkali metal with a favorable absorption cross section such that tenuous gases are easily illuminated at select transiting exoplanet systems. We examine both the time-averaged and time-series alkali spectral flux individually, over 4 nights at a hot Saturn system on a ∼2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO observations are analyzed, providing new constraints. We recover the previously confirmed residual sodium flux uniquely when averaged, whereas night-to-night Na I varies by more than an order of magnitude. On HARPS/3.6 m Epoch II, we report a Doppler redshift at vΓ,NaD = + 9.7 ± 1.6 km s‑1 with respect to the planet's rest frame. Upon examining the lightcurves, we confirm night-to-night variability, on the order of ∼1%–4% in NaD, rarely coinciding with exoplanet transit, not readily explained by stellar activity, starspots, tellurics, or the interstellar medium. Coincident with the ∼+10 km s‑1 Doppler redshift, we detect a transient sodium absorption event dFNaD/F⋆ = 3.6% ± 1% at a relative difference of ΔFNaD(t) ∼ 4.4% ± 1%, lasting ΔtNaD ≳ 40 minutes. Since exoplanetary alkali signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly ∼‑5.7 km s‑1, the radial velocity is rather at +15.4 km s‑1, far larger than any known exoplanet system. Given that the redshift magnitude vΓ is in between the Roche limit and dynamically stable satellite orbits, the transient sodium may be a putative indication of a natural satellite orbiting WASP-49 A b.
Keywords
Natural satellites (Extrasolar); Exoplanet astronomy; Transmission spectroscopy; Radial velocity; Doppler shift
The Astrophysical Journal Letters
Volume 973, Number 2
2024 October
